1. Field of the Invention
This invention relates to the field of semiconductor processing and, more particularly, to an alternative method for forming an interlevel dielectric comprising borophosphosilicate glass or boro-phospho-tetra-ethyl-ortho-silicate using boron implantation.
2. Description of Relevant Art
It is well known that alpha particles are very harmful to the operation of semiconductor devices and, more particularly, semiconductor devices that are charge-sensitive. Such devices may include dynamic random access memories ("DRAMs"), charge-coupled devices ("CCDs"), and static random access memories ("SRAMs"). An alpha particle comprises a high energy, positively charged helium nucleus. As the alpha particle travels through the device, the amount of charge present in the device changes. In the case of a memory device, this may mean changing the charge enough to the point of changing the status of the device. Such errors caused by alpha particles are generally referred to as soft errors. Therefore, limiting the production of alpha particles is essential in ensuring reliable charge-sensitive devices.
A major alpha particle source is boron-10, one of the two stable isotopes of boron. Naturally existing boron comprises 20% boron-10 and 80% boron-11. An alpha particle is created when a cosmic ray neutron strikes a boron-10 nucleus to produce a lithium-7 atom and a high energy alpha particle. That is: EQU .sup.10 B+.sup.1 n.fwdarw..sup.7 Li+.sup.4 .alpha..
Boron-10 has a thermal neutron cross-section of 3838 barns (3.838.times.10.sup.-21 cm.sup.2); most elements have a thermal neutron cross-section of about 10.sup.-3 barns (10.sup.-27 cm.sup.2). Therefore, the probability that a boron-10 nucleus will absorb a cosmic neutron and emit a high-energy alpha particle is very high.
One way to overcome soft error problems associated with boron-10 is to avoid using it in device processing. Boron may be purchased as pure boron-11 from a supplier and can be used in all process applications in the place of the naturally occurring mixed boron. Purchasing pure boron-11, as opposed to naturally occurring boron, increases production costs. Since boron-11 and boron-10 have identical chemical properties, using pure boron-11 during processing should have no effect on device performance. Boron is used throughout modern semiconductor devices including source regions, drain regions, gate structures, and interlevel dielectrics. Boron concentration in interlevel dielectrics is much higher than boron concentrations in the source regions, the drain regions, and the gate structure. Therefore, the interlevel dielectric is the most critical area for reducing the presence of boron-10.
Borophosphosilicate glass ("BPSG") is used as an interlevel dielectric to provide insulation between the overlying metal layer and the underlying polysilicon gates, source regions, and drain regions. BPSG is formed in a single-step process using chemical-vapor deposition ("CVD"). A layer of silicon nitride is sometimes first formed upon the device to protect the polysilicon, source and drain regions, and the salicides from dopants inside the interlevel dielectrics. An interlevel dielectric needs to have good flow characteristics so that it can fill in gaps over the semiconductor substrate and give rise to a fairly planar upper surface. After deposition, the wafer is heated and the interlevel dielectric flows to fill in existing gaps and become planar. In addition, the heat treatment helps activate source and drain implants. An interlevel dielectric must also have good reflow properties. After contact holes are etched into the interlevel dielectric for subsequent metal deposition, the interlevel dielectric is heated which causes it to reflow. Consequently, the previously vertical sidewalls of the holes are now sloping, and the previously sharp corners are now smooth. This smoothing aids subsequent metal deposition by allowing the metal to reach the underlying silicon more easily and without creating voids. BPSG is a ternary (three-component) glass, made out of B.sub.2 O.sub.3 --P.sub.2 O.sub.5 --SiO.sub.2. Phosphorus is added to the dielectric to reduce stress and, to a lesser degree, improve step coverage. A concentration of at least 6 wt % phosphorus is needed for the dielectric to flow. Also, the addition of phosphorus makes the interlevel dielectric an excellent getter of sodium atoms. Sodium contamination can cause instability with the threshold voltage. Boron is added to make the glass flow at lower temperatures. Making the glass flow at lower temperatures helps conserve the thermal budget. The more boron added the more flowable the glass is. However, boron concentrations above 5 wt % make the glass unstable. BPSG with approximately 5 wt % phosphorus and 5 wt % boron concentrations can be made to flow around 800.degree. C. or less, much lower than temperatures needed for undoped glass. Adding boron to the BPSG during the CVD process produces an upper surface of the BPSG with "boron bumps". An extra step for removing the bumps is then required. A mechanical scrubbing process is preferably used to remove the bumps and planarize the upper surface of the interlevel dielectric. The extra processing step decreases throughput and increases production costs.
Boron is essential in making interlevel dielectrics able to flow at approximately 800.degree. C. or less. Furthermore, if boron-11 is used exclusively, soft errors are substantially eliminated since production of alpha particles is minimized. Therefore, it would be desirable to be able to form a boron-bearing interlevel dielectric which is free of "boron bumps" on the upper surface of the interlevel dielectric. Furthermore, it would be desirable to be able to introduce boron-10 or boron-11 into the dielectric without having to purchase boron in either pure boron-10 or pure boron-11 form from a supplier. Separation of boron into its isotopes during the production process would significantly reduce production costs.